A. Field of the Invention
Broadly speaking, this invention relates to optical fibers. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for manufacturing an optical fiber having azimuthal index inhomogeneities.
B. Discussion of the Prior Art
The advantages of deliberately enhancing mode conversion in a multi-mode optical fiber have been widely reported in the literature. Typically, this enhancement is achieved by randomly modulating the cross-sectional dimensions and axial orientation of the guide. See, for example, U.S. Pat. Nos. 3,687,514 and 3,666,348 which respectively issued Aug. 29, 1972 to S. E. Miller et al. and on May 30, 1972 to E. A. J. Marcatili. Both of these commonly assigned patents are hereby incorporated by reference into this specification.
Recent analytical studies by D. Marcuse have suggested the desirability of introducing azimuthal index variations into the guide to mix modes of different azimuthal order. These studies further suggest that any resultant radial index inhomogeneities (which mix radial orders) be confined to a region which is close to the axis of the core. However, heretofore it has not been possible to implement either of Marcuse's proposals.
There are, of course, several different techniques which may be used to manufacture a clad optical fiber, including the so-called chemical vapor depositiion (CVD) process disclosed in U.S. Pat. Nos. 3,711,262 -- 3,737,292 and 3,737,293. Of particular interest is the commonly assigned, copending application, MacChesney-O'Connor, Ser. No. 444,705, filed Feb. 22, 1974, which application is hereby incorporated by reference. As disclosed in the MacChesney-O'Connor application, a constantly moving stream of gas-phase precursor reactants and oxygen is flowed through a hollow tubular glass preform. The preform and its contents are then heated to a homogeneous reaction temperature within a moving hot zone produced by a moving heat source which constantly traverses the outside of the tube. Homogeneously produced glass particles, loosely called "soot," collect on the walls of the tube and are fused into a continuous layer within the hot zone. The preform is then collapsed and drawn into a fiber in the normal manner. This process differs from the prior art because in prior art processes a simultaneous heterogenous reaction occurs which produces a glassy layer within the moving hot zone by means of a reaction which occurs at the heated wall surface.